86 MB. C. DAB,WI>' 0:S THE DlilOHPHIC COXBITIOS 



and in falling off do not brush over the lowly seated stigma. In 

 the Cowslip the coroUa does not fall off; and both long-styled and 

 short-styled plants are equally sterile when protected from insects. 

 It is a rather cvirious case, that the falling of the corolla, or its re- 

 maining attached when withered, might have a considerable in- 

 fluence on the numbers of a plant, during a year unfavourable to 

 the visits of the proper insects. 



In three short-styled plants of Primula auricula, protected from 

 insects, the flowers which I fertilized produced seed, but those 

 which were not touched produced none. 



In all the species of Primula the pollen readily coheres to any 

 object. In all that I have observed, though the stamens and pis- 

 tils differ in length relatively to each other in the different species, 

 yet, in the two forms of the same species, the stigma of the one 

 form stands at exactly the same height with respect to the corolla 

 as the anthers of the other form. If the proboscis of a dead 

 Humble-bee, or thick bristle, or rough needle be pushed down the 

 corolla, first of one form, and then of the other, as an insect would 

 do in visiting the two mingled forms, it will be found that pollen 

 from the long-stamened form will adhere round the base of the 

 proboscis, and will be left with certainty on the stigma of the long- 

 styled form ; pollen from the short stamens of the long-styled 

 form wOl also adhere a little above the tip of the proboscis, and 

 some wiU generally be left on the stigma of the other form. Thiis 

 pollen will be carried lecipTocally from one form to the other. In 

 withdrawing the proboscis from the long-styled form, with poUen 

 adhering near the tip, there will be a good chance of some being 

 left on the flower's own stigma, in which case there wiU he self- 

 fertilization ; but this by no means always occurs. In the short- 

 styled form, on the other hand (and it is important to remember 

 this), in inserting the proboscis between the anthers situated at 

 the mouth of the corolla, poUen, as I repeatedly found, is almost 

 invariably carried down and left on the flower's own stigma. 

 Moreover minute insects, such as Thrips, numbers of which I have 

 observed in Primrose flowers thickly dusted with pollen, could not 

 fail often to cause self-fertilization. We positively know that the 

 visits of large insects are necessary to the fertilization of the species 

 of Primula ; and we may infer from the facts just given that these 

 visits would carry pollen reciprocally from one form to the other, 

 and would likewise tend to cause self-fertilization, more especially 

 in the short-styled (i. e. long-stamened) form. 



These observations led me to test the potency of the two pol- 



